Marsupials: A southern
success story

Marsupials are often associated with the Australian continent
where such popular animals as kangaroos and koalas live. Less
widely known is that a number of marsupials occur in South
America today. These include the opossums, the only marsupials
that also occur in the northern hemisphere today.

Surprisingly, Mesozoic marsupials are mainly known from North
America, and they may have originated there during the
Cretaceous. Primitive opossum-like marsupials are among the most
common and most diverse mammals in the late Cretaceous of North
America. They include cat-sized forms like Didelphodon,
one of the largest Mesozoic mammals - and one of the most widely
known thanks to its appearance in "Walking with
Dinosaurs". Didelphodon had teeth specialized for
crushing, and it has been suggested that it fed on hard shelled
food like clams, snails, turtles or even young ankylosaurs.

The diversity of North American marsupials was dramatically
reduced at the end of the Cretaceous. Only one single lineage,
represented by late Cretaceous Alphadon and Paleocene Peradectes,
survived the faunal turnover. Thus marsupials fared hardly better
than dinosaurs in North America, and worse than reptiles on the
whole. Peradectes survived into the Eocene, and a few
other opossum-like marsupials appear in North America during the
Paleocene. However, marsupials never became a significant
component of northern faunas again and finally became extinct
there. Opossums reentered North America from South America only a
short time ago, after the Isthmus of Panama had formed in the
Pliocene.

Figure 1: Reconstruction of Late Cretaceous Alphadon,
based mainly on the appearance of todays opossums since only the
dentition of this animal is known. This reconstruction should be
equally valid for Peradectes,
the Paleocene to Eocene successor of Alphadon.
From Cox (1988).

Marsupials were much more succesful on the southern
continents. They arrived in South America relatively late in the
Cretaceous, but by early Paleocene time this group of mammals was
flourishing there. Marsupials make up more than 50% of the mammal
species in some South American Paleocene faunas, and adaptive
types include insectivores, omnivores, carnivores and small
herbivores. Paleocene mammals are not yet known from Antarctica
and Australia. However, these continents were connected to each
other and to South America at that time, and Eocene marsupials
occur there. So it is highly probable that marsupials lived there
during the Paleocene as well.

Many Paleocene marsupials of
South America are closely related to todays opossums, which are
appropriately called "living fossils". Opossums and
their relatives belong to the order Didelphimorphia (named after
the recent Virginia opossum Didelphis). Pucadelphys,
a primitive opossum from the early Paleocene of Bolivia, is known
from the best fossils for any early marsupial. Nearly complete
skeletons have been found in pairs, which are interpreted as male
and female. They were probably trapped in their burrows during a
flood. Pucadelphys was an agile animal with some
capabilities for bounding and digging, but without strong
adaptations for climbing in trees. Like most recent opossums it
was probably an omnivore that fed on insects, fruits and small
vertebrates.

Figure 2: Pair of skeletons of the opossum Pucadelphys
andinus from the early Paleocene of
Bolivia, preserved in a life-like way just as resting in their
nest. Skeleton on top of the figure faces to the right, skeleton
at the bottom (without skull) to the left. From Rich & Rich
(1993).

Abundant fossil Didelphimorphia are known from middle
Paleocene fissure fillings in Brazil. Members of this order cover
a large range of size and adaptation at that time. Minusculodelphis
was close in size to the smallest known mammals. It is based on
fossil jaws that contain teeth less than a millimeter in size!
This tiny animal probably hunted for insects. Eobrasilia
was much larger and had bulbous teeth similar to Late Cretaceous Didelphodon.
It was previously considered as a relative of the carnivorous
Sparassodonta discussed below, but is now regarded as an
independent branch of opossum relatives that evolved towards a
more carnivorous niche.

A second group of marsupials,
the order Paucituberculata, is known in South America from the
Paleocene to the present. During the Paleocene the most important
members were the extinct polydolopids, rat to rabbit-sized
animals that were somewhat similar to rodents in ecology. The
middle Paleocene genus Epidolops is known from a nearly
complete skull. All other polydolopids are only known from fossil
jaws and teeth. Like rodents, polydolopids had enlarged front
teeth. Still more distinctive was that some of their cheek teeth,
especially the last premolars, formed large blades for cutting
food. Similar blade-like cheek teeth have evolved independently
in several groups of mammals, for instance the Mesozoic to early
Tertiary multituberculates and todays
rat-kangoroos. Such teeth could be useful for processing tough
parts of plants, but an animal with this dentition could be an
omnivore as well. Polydolopids declined in diversity during the
Eocene and became extinct in the Oligocene, probably due to
competition with other groups of Paucituberculata and with
immigrating rodents and primates.

Figure 3: Lower jaw of the polydolopid Epidolops
ameghinoi from the middle Paleocene of
Brazil. Note the large front teeth and the blade-like last
premolar. Later polydolopids like Polydolops
developed strong serrations on this tooth. From Paula Couto
(1952).

Finally, a group of marsupials
started during the Paleocene to occupy the ecological role of
carnivorous mammals in South America. These animals are
classified in the extinct order Sparassodonta, which culminated
in the dog- or bear-like borhyaenids and the sabre-toothed
cat-like thylacosmilids of the later Cenozoic. Paleocene members
of this order demonstrate the transition from primitive
opossum-like animals such as Pucadelphys to ferocious
predators. Mayulestes from the early Paleocene of Bolivia
was a small predator which occupied a ecological niche close to
that of weasels or martens. Adaptations like a prehensile tail
show that this agile animal lived partially in trees. On the
ground Mayulestes was probably capable of relatively fast
but short runs to catch its prey.

The first larger sparassodonts are known from the middle
Paleocene of Brazil. They include the medium sized genus Patene
(meaning 'fox' in a native South American language) and a poorly
known wolf-sized predator. These marsupial carnivores developed
basically the same tooth structure for cutting meat as the true (placental)
carnivores of the northern hemisphere. Unlike true
carnivores, however, sparassodonts did not develop particular
adaptations for fast running. This may have been one reason for
their decline and finally their extinction after the arrival of
true carnivores in South America in late Cenozoic time.